Jeffrey Andrews-Hanna

We see a lot of straight sides, we see sharp angles, right angle corners here.

There is a giant square on the near side of the moon, almost 2,000 kilometers across, and we didn't know about it before we had GRAIL data in hand.

That to me is pretty impressive, but from the standpoint of an impact, well, that nice circular basin that we were looking for

what we see is a square.

And the problem is that impact craters, generally speaking, aren't square.

They generally don't have straight sides.

They typically don't have corners.

As an example, you could argue, oh, but when things get big, all bets are off, right?

But this is a giant impact basin on Mars called the Borealis Basin.

This is topography and kind of a model of crustal thickness.

And it's round.

It's elliptical, just like most basins are, so most basins are actually more elliptical than circular, but they've got a pretty consistent size and shape.

Prossal Arm is quite a bit smaller than that, and it's a giant rectangular square.

So this really doesn't match what we expected to see in looking for an impact basin, and so we want to look for other hypotheses to try to explain it.

Now, this is still a big puzzle.

What we can do with gravity data, we can try to model the subsurface structure.

This area, as I said, has been flooded with a lot of lava.

Lava igneous rocks tend to be more dense.

And so if we want to look at the structure, we want to look at both the thickness of that low-density crust as well as the thickness of the lava.

And we have to take both of those into account.